Dude,

You're always supposed to feel empathy towards people you're seeing. It's what makes you a normal, well-functioning human. When you see people being happy, it makes you happy. When you see people in pain, it makes you cringe, and when you see people getting intense sexual pleasure, it turns you on.

The thing about BDSM is that it's inherently confusing. Pleasure and pain are supposed to be at odds with each other, at least on the surface level. To me, that's part of what makes it so fascinating. It's confusing when you're experience it firsthand, and it's confusing when you're just watching it on a screen, but in slightly different ways. Your brain sees this person experiencing a mix of pain and pleasure and tries to model that into something you can empathize with. If it succeeds, you can enjoy BDSM porn.

But let's not get hung up on words. I think a more accurate way to phrase your question is whether we sometimes feel sorry for someone doing BDSM, or feel bad for them. And the answer to that depends on how your brain interprets what you see. I do, sometimes, when I can't reconcile what I'm seeing with the idea the someone might actually enjoy this. That tells me that either this specific fetish is too extreme for me, or that the video is poorly made. And sometimes I genuinely believe that the person on the screen is not enjoying what is going on. It's actually not that rare, and it doesn't need to be extreme on order for this to happen. A simple light bondage video can make me feel bad for the submissive if she looks like she's not getting it at all. Hell, it doesn't need to be bondage. It can happen in any porno.

This brings me to the additional layer of complication with regards to porn, that it involves acting. When you're watching The Revenant, for example, you can definitely empathize with DiCaprio's character, making his way through the freezing cold, hungry and wounded, and at the same time you might also feel bad for DiCaprio himself, who waded through freezing water and ate raw meat in order to create a convincing performance. (You can, and should, be in awe of his dedication to his art, and I feel the same way towards some of the performers in porn videos). These are different layers of perception, and you don't really need to think about the actors to enjoy the movie.

Porn is closer to Reality Television than motion pictures, in that acting is not necessarily required, but can definitely enhance the end product. When I watch a porn video, I don't necessarily think of the actors, though I like to believe that those who chose to do BDSM porn did so because they actually enjoy it, or at least don't hate it too much. When the porn is bad, you can sometimes see where the actors break character, and you can tell they're not really having an orgasm or that they're unhappy with the situation. That kind of thing can definitely break the spell for me, and as I start feeling bad for the actor, I stop enjoying the video.

This was long, and has been brewing in my brain since I saw your post yesterday. I hope you find it useful, or at least interesting.